Fiber reinforced plastics (FRP) are light in weight and have superior mechanical properties and therefore are used widely for electrical or electric instrument applications, civil engineering or building applications, machine or mechanical component applications, robot applications, motorcycle or automobile applications, universal or aerial applications, etc. As reinforcing fibers to be used for such FRP, there have been used metal fibers, such as aluminum fibers and stainless steel fibers, organic fibers, such as aramid fibers and PBO fibers, inorganic fibers, such as silicon carbide fibers, and carbon fibers. Among these, carbon fiber is preferably used from the viewpoint of being excellent in specific strength and specific rigidity and being capable of affording excellent lightness.
Here, one example of representative FRP like carbon fiber-reinforced plastics (CFRP) is a molded product produced by subjecting a preform obtained by laminating prepregs to press molding (a molding method comprising defoaming and shaping performed under pressurization). Prepregs are commonly produced by a method comprising impregnating a reinforcing fiber base material prepared by arraying in one direction or weaving continuous reinforcing fibers with a resin.
Superior mechanical properties can be obtained with molded products prepared by using such prepregs. On the other hand, since reinforcing fibers are used in their original continuous form, they are unsuitable for shaping into a complicated shape. Moreover, since the lamination angle of prepregs has a great influence on properties, it is necessary to laminate prepregs by paying attention of the lamination angle. In other words, since a lamination step requires time and effort and the cost increases accordingly (i.e., an economic burden caused by the lamination step will increase), usage is restricted.
Patent document 1 proposes a prepreg that is effective for shaping into a complicated shape by cutting reinforcing fibers into a specific length. However, since a lamination step requires time and effort like that mentioned above, an economic burden has not been eliminated.
On the other hand, FRPs using discontinuous reinforcing fibers have also been proposed. Sheet molding compounds (SMC) and glass mat base materials (GMT) are materials suitable for press molding. However, their usage is restricted because their mechanical properties, such as specific strength and specific rigidity, are poor, they are difficult to be applied to thin molded products, and isotropic mechanical properties are hardly obtained because resin flows very much at the time of molding, and their characteristics vary widely.
Patent documents 2, 3 each propose a sheet material in which more isotropic properties can be obtained by dispersing reinforcing fibers in a bundle form. In patent document 4 is proposed a sheet material with superior mechanical properties caused by uniform dispersion of carbon fibers. However, since all of them cannot be processed to be thin like a prepreg and resin flows greatly at the time of shaping, an isotropic property may be impaired, and mechanical properties may also decline.
Moreover, in patent document 5 is proposed a molded product in which carbon fibers have been fixed while being randomly dispersed in the form of single yarn. Also in this method, there is a limit in processing it to be thin and therefore the degree of freedom of lamination of a preform is restricted. Furthermore, since it is impossible to produce a large number of preforms, an economic burden has not been eliminated.